Hidden pairs refer to a pair of numbers that only appear in two cells. For example, consider the following row of 9 cells and the various numbers that are possibilities for each cell;

12 356 1457 26 1478 189 356 569 235

In this example, there is a hidden pair of 4 and 7 in cell number 3 and cell number 5. They can only be a hidden pair if both numbers are not to be found as possibilities in any other cell. So, where they are useful is in removing other numbers as possibilities. In the above example, it is logical that the numbers 4 and 7 must appear in cell numbers 3 and 5.
Therefore, you can exclude the numbers 1 and 5 from cell number 3 and the numbers 1 and 8 from cell number 5.